1. Field of the Invention
This invention relates to the removal of impurities from a gas stream, and more particularly to the removal of carbon dioxide from a gas stream. The invention is particularly applicable to the removal of carbon dioxide from an ambient air stream prior to introduction of the air stream into a conventional cryogenic air separation unit.
It is often desirable or necessary to remove certain impurities, such as carbon dioxide and moisture, in the form of water vapor, from a gas stream prior to further processing of the gas stream. For example, conventional air separation units (ASUs) for the production of nitrogen and oxygen by the cryogenic separation of air are basically comprised of a two-stage distillation column which operates at very low temperatures. Due to the extremely low temperatures, it is essential that water vapor and carbon dioxide be removed from the compressed air feed to an ASU. If this is not done, the low temperature sections of the ASU will freeze up making it necessary to halt production and warm the clogged sections to revaporize and remove the offending solid mass of frozen gases. This can be very costly. It is generally recognized that, in order to prevent freeze up of an ASU, the content of water vapor and carbon dioxide in the compressed air feed stream must be less than 0.1 ppm and 1.0 ppm, respectively.
A process and apparatus for the pre-purification of a gas must have the capacity to constantly meet, and hopefully exceed, the above levels of contamination and must do so in an efficient manner. This is particularly significant since the cost of the pre-purification is added directly to the cost of the product gases of the ASU.
2. Description of the Relevant Prior Art
Current commercial methods for the pre-purification of gases include reversing heat exchangers, temperature swing adsorption and pressure swing adsorption. The first two of these approaches are described by Wilson et al. in IOMA BROADCASTER, Jan.-Feb., 1984, pp 15-20.
Reversing heat exchangers remove water vapor and carbon dioxide by alternately freezing and evaporating them in their passages. Such systems require a large amount, i.e. 50% or more, of product gas for the cleaning, i.e. regenerating, of their passages. As a result of this significant disadvantage, combined with characteristic mechanical and noise problems, the use of reversing heat exchangers as a means of pre-purification has steadily declined over recent years.
In temperature swing adsorption (TSA) pre-purification, the impurities are removed at low temperature, typically at about 5.degree. C., and regeneration is carried out at elevated temperatures, e.g. from about 150.degree. C.-250.degree. C. The amount of product gas required for regeneration is typically only about 12%-15%, a considerable improvement over reversing heat exchangers. However, TSA processes require both refrigeration units to chill the feed gas and heating units to heat the regeneration gas. They are, therefore, disadvantageous both in terms of capital costs and energy consumption.
Pressure swing adsorption (PSA) processes are an attractive alternative to TSA since both adsorption and regeneration are carried out at ambient temperature. PSA processes, in general, do require substantially more regeneration gas than TSA which can be disadvantageous when high recovery of cyrogenically separated products is desired. This disadvantage can be substantially reduced, however, in a cryogenic plant which has a substantial waste stream, e.g. about 40% of the feed. Such streams are ideal as regeneration gas since they are impurity free, i.e. free of water vapor and carbon dioxide, and would be vented in any event. However, although many pre-purification methodologies based on PSA have been proposed in the literature, few are actually being used commercially due to high capital and energy costs associated therewith.
German Patent Publication DE 3,045,451 (1981) describes a PSA pre-purification process which operates at 5.degree.-10.degree. C., 880 KPa (9 Kg/cm.sup.2) adsorption pressure and 98 KPa (1 atm) regeneration pressure. Feed air is passed under pressure through a layer of 13X zeolite particles to remove the bulk of water vapor and carbon dioxide and then through a layer of activated alumina particles to remove the remaining low concentrations of carbon dioxide and water vapor. It is stated that the secondary layer of activated alumina can comprise from about 20%-80% of the combined volume of the bed. The arrangement of the adsorbent layers in this manner is claimed to reduce the formation of "cold spots" in the adsorbent beds.
A process similar to that of this German Patent Publication is discussed by Tomomura et al in KAGAKU KOGAKU RONBUNSHU. 13(5), (1987), pp 548-553. This latter process operates at 28.degree.-35.degree. C., 0.65 MPa adsorption pressure, and 0.11 MPa regeneration pressure, has a sieve specific product of 7.1 Sm.sup.3 /min/m.sup.3 of sieve and a vent gas loss of 6.3% of the feed air. This is the amount of additional air that would have to be compressed to make up for the vent gas loss. While 6.3% would appear to be a relatively low number, each one percent by volume of feed air lost in the vent represents, on the average, an annual operating loss of ten thousand dollars for a plant producing two hundred tons of nitrogen per day.
Japanese Kokai Patent Publication Sho 59-4414 (1984) describes a PSA pre-purification process in which separate beds and adsorbents are used for water vapor and carbon dioxide removal. The water vapor removal tower containing activated alumina or silica gel is regenerated by low pressure purge while the carbon dioxide removal tower containing 13X zeolite is regenerated by evacuation only without a purge. This process requires about 25% regeneration gas and, as a result, would be used with regard to cryogenic processes having a high product recovery. However, where the cryogenic plant produces a substantial waste stream, such processes are expensive because of the power demands of the vacuum pump.
Japanese Patent Publication Sho 57-99316 (1982) describes a process wherein feed air, vent gas and purge gas are passed through a heat exchanger to thereby cause adsorption and desorption to take place at nearly the same temperature. The advantage of this process is stated to be a reduction in the required quantity of regeneration gas.
In the process described in Japanese Patent Publication Sho 55-95079 (1980), air is treated by PSA in two stages to remove water vapor and carbon dioxide wherein dry air product from the PSA unit is used to purge the first stage and an impure nitrogen stream from the ASU is used to purge the second stage. This process is stated to be advantageous in terms of the overall nitrogen recovery.
U.S. Pat. No. 4,711,645 describes a pre-purification PSA process utilizing activated alumina for removal of water vapor and a zeolite for carbon dioxide removal. It is stated that the use of activated alumina for water removal allows adsorption at a lower temperature and, therefore, carbon dioxide adsorption takes place at a lower temperature. Both adsorption and desorption take place at close to ambient temperature.
In the PSA cycle described in laid-open German Offen. DE 3,702,190 A1 (1988), at least 80% of the heat of adsorption is retained in the bed and is available for regeneration. The process of this patent document includes the use of initial bed of silica gel or alumina for moisture removal and a second bed of 13X zeolite for carbon dioxide removal. The principle of retaining heat of adsorption in PSA beds is well established in the art.
Most of the prior art PSA air purification processes, with the exception of the German Patent Publication DE 3,045,451, utilize an initial bed or layer containing activated alumina or silica gel for water vapor removal and then a zeolite bed or layer for carbon dioxide removal. German Patent Publication DE 3,045,451 utilizes zeolite particles to adsorb the bulk of the water vapor and carbon dioxide present and then utilizes a layer of activated alumina to remove low concentrations of both impurities that remain from the first bed.
In accordance with the present invention, a means of efficiently removing water vapor and carbon dioxide has been found which is advantageous over the prior art in terms of power consumption and vent gas loss.